n-channel depletion MOS devices were irradiated with 50 and 80 MeV phosphorous ions, with different fluences varying in the range from 1011to 1013ions/cm2. The pre and post irradiation I-V characteristics were measured and the corresponding threshold shift ΔVTHwas estimated. In both the cases, the drain current IDand the threshold voltage VTHwere found to decrease with the ion fluence. The increase in the threshold voltage shift ΔVTHwith the ion fluence, was greater for the devices irradiated with 80 MeV ions than those irradiated with 50 MeV ions. The interface and oxide state densities were determined through the subthreshold voltage measurements. To separate the contributions of oxide and interface states towards the threshold voltage shift, the ion irradiated MOS devices were annealed at 150 °C. The threshold shift during annealing initially decreased and later increased with increasing annealing period. The rate of change of the interface states during annealing was higher than that of the oxide states. It was also found that depletion mode (normally ON) MOSFETs switched operation to enhancement mode (normally OFF).
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